Fibrous carbon or graphite products and method of making same

ABSTRACT

A HIGH-STRENGTH HIGH-TEMPERATURE, ABLATION-RESISTANT COMPOSITE OR STRUCTURE IS FORMED OF FIBROUS CARBON OR GRAPHITE BONDED IN A CARBONIZED BINER. THE FIBROUS STRUCTURE IS FORMED OF SMALL, STRAIGHT, CARBONACEOUS FIBERS OF VIRTUALLY UNIFORM LENGTH WHICH ARE ADMIXED WITH A CARBONIZABLE BINDER AND SPRAYED ONTO A SUITABLE SURFACE. THE SPRAYED ADMIXTURE IS THEN HEATED SUFFICIENTLY TO CARBONIZE OR GRAPHITIZE THE BINDER FOR FORMING A HIGH-STRENGTH, CARBONACEOUS STURCUTRE OF DESIRED CONFIGURATION. THE FIBERS IN THE CARBONACEOUS STRUCTURE ARE RANDOMLY ORIENTED AND UNIFORMLY DISPERSED TO PROVIDE THE FIBROUS STRUCTURE WITH THE AFOREMENTIONED PROPERTIES.

24a atsratoee 3,573,086 FIBROUS CARBON R GRAPHITE PRODUCTS AND METHOD OF MAKING SAME Foraker Lambdin, Jr., Alcoa, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Dec. 10, 1968, Ser. No. 782,714

. Int. Cl. B44c 1/08, 1/00 US. Cl. 117-46 11 Claims ABSTRACT OF THE DISCLOSURE the carbonaceous structure are randomly oriented anduniformly dispersed to provide the fibrous structure with the aforementioned properties.

The present invention relates generally to fibrous composites and more particularly to an improved form of fibrous carbon or graphite composites or structures and a method of making the same. This invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

High-temperature applications such as space reentry vehicles, rocket nozzles, combustion chamber liners, heat shields, etc., require structural materials which exhibit high strength, resistance to thermal shock, and good resistance to erosion by ablation. Considerable success has been achieved with carbonaceous materials; i.e., carbon and graphite reinforced with fibers of glass, asbestos, or carbonized and graphitized products. The use of carbon or graphite fibers as the reinforcing material has proven to be particularly desirable since these carbonaceous fibers are sufiiciently light and flexible to permit their usage in composites of various configurations. Further, the strength characteristics of these fibers improve with increasing temperature.

Composites of fibrous carbon or graphite were previously produced by macerating or shredding organic textiles, which had been previously carbonized or were readily carbonized or graphitized, intofibers of random.

lengths, usually in the order of about to 250 mils. These fibers were admixed with a resinous binder, formed into the desired shape (often under pressure), and thereafter carbonized or graphitized.

In spite of the fact that the previously known fibrous carbon or graphitecomposites of the type described above have contributed significantly to satisfyingthe need for a high-strength and ablation-resistant material, there are shortcomings or drawbacks which significantly detract from the usefulness of these previously known fibrous materials in the envisioned applications. For example, the use of maceratcd or shredded fibers resulted in considerable fiber alignment and agglomerations of the aligned fibers which in turn caused weak areas in the structure due to the lack of or too many fibers and also because the aligned fibers in the composite created minimal strength characteristics along planes parallel to the planes of the aligned fibers. Further, wide and unacceptable variations in the thickness of fibrous composites resulted when the latter were prepared by spraying a resinous mixture containing macerated fibers onto a form.

It is the aim of the present invention to obviate or minimize the above and other shortcomings or drawbacks by providing new and improved forms of fibrous carbon or graphite composites. 'Each of these composites enjoys a high strength-to-density ratio, high-temperature strength, resistance to thermal shock and ablation by high-temperature fluids, and compressive and tensile strengths which are virtually,.uniform in all three directions. The fibrous carbon or graphite composites of the present invention are produced by admixing discrete, very small carbon or. graphite fibers of uniforrrrlength and straightness with a suitable carbonizable binder, e.g., a thermosetting or thermoplastic resin; forming a fiber resin composite by air-spraying the mixture onto a suitable surface of the desired configuration; and thereafter carbonizing or graphitizing the binder. Fibrous composites produced in accordance with this invention attained fibers which are randomly oriented and uniformly dispersed throughout the composite or structure so as to provide the aforementioned unique or desirable properties and characteristics.

An object of the present invention is to provide a new and improved form of fibrous carbon or .graphite.

Another object of the present invention is to provide fibrous carbon or graphite composites wherein the carbon or graphite fibers are of virtually equal length and are uniformly dispersed and randomly oriented throughout the composite to instill in the composite high-strength properties in all three directions as well as high-temperature strength and resistance to thermal shock and ablation.

A further object of the present invention is to fabricate fibrous carbon or graphite composites by air-spraying an admixture of a carbonizable binder and carbon or graphite fibers or uniform length and straightness onto a suitable form and thereafter carbonizing or graphitizing the binder in a sprayed deposit.

A still further object of the present invention is to provide a fibrous carbon or graphite composite having a density range of about 0.2 to 1.7 gms./cc. and of a thickness ranging from a thin 7-rnil layer upwards to 2 to more inches.

Other and further objects of the invention will be obvious-upon an understanding of the illustrative data and examples about to the described, or will be indicated in.

the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

Described generally, the fibrous carbon or graphite composites of the present invention are formed of discrete, very small, carbonaceous (carbon or graphite) fibers of virtually uniform length and straightness that are randomly oriented and uniformly dispersed in a matrix provided by a carbonized or graphitized binder. The

manufacture of these composites comprises the steps of admixing individual chopped fibers which have been previously converted to carbon or graphite form with a car-- bonizable binder, spraying the mixture onto a mandrel of desired shape to form a structure of near-finished dimensions, and thereafter heating the structure in an inert atmosphere to a temperature sufficient to Carbonize or graphitize the binder. Normally, a temperature of about 900 to 1000 C. is sufiicient to carbonize the binder,

'while a temperature in the range of about 2500 to 3000 C. may be used tographitize the binder. Either the carbon or graphite fibers may be used in the composite having a carbonized binder, but only the graphite fibers may be satisfactorily employed ina composite in which the Patented Mar. 30, 1971 l provide a desirable high strength-to-density ratio characteristic in the composite.

In-the production of the fibrous composites of the pres ent invention it has been found that the random orientation and the, uniformity of the dispersion of fibers in the composite increase or improve with the decrease in the length. These fibers must be straight and virtually uniform in length to prevent agglomeration or clumping together of the fibers. While the carbon or graphite fibers may be produced from several materials, some success has been achieved with high-twist graphite yarn cut to lO-mil lengths. These fibers are preferably screened on a 30mesh screen (Tyler) to assure separation or classification of the various fibers of uniform length. Less than 1 percent of the cut yarn remains on the 30-mesh screen after the sieving operation so as to provide fibers which can be sprayed onto a form without agglomerating. Low-twist graphite yarn, on the other hand, cut in the same way yielded only about 80 wt. percent of the yarn less than 30 mesh and this material was agglomerated and could not be sprayed with any acceptable degree of success. While the high-twist graphite yarn has proven to be a satisfactory source of graphite fibers for the purposes of the a binder-tofiber ratio of about 60-l00 parts per hundred can be satisfactorily employed in the production of the carbonaceous composites of the present invention.

present invention, it is relatively expensive and in short supply. Successful results have also been achieved by employing carbon or graphite fibers produced from cellulosic products such as rayon or cotton. These products are cut or chopped to desired lengths and then carbonized or graphitized prior to admixture with the carbonizable binder. Rayon yarn of 2.29 denier (-18 microns) chopped to IS-mil lengths is used to produce graphite fibers mils in length and 5 mils in diameter and rayon yarn of 0.15 denier (-3.7 microns) chopped to the same length or less is used to produce graphite fibers in the diameter range of l to 2 microns. For producing carbon fibers 1.5 microns in diameter by 8-10 mils in length, 3.7-micron rayon yarn of l5-mil lengths is utilized. After the organic or synthetic materials are chopped in the particular length desired for the fibers, carbon fibers may be formed by carbonizing the material in an inert atmosphere at a temperature of about 900l000 C. and then, if desired, converted to graphite by heating the carbon fibers at a temperature of about 3000 C. in an inert atmosphere. The temperatures at which the fibers are carbonized or graphitized should be at least as great as the temperatures employed in carbonizing or graphitizing the binder used in the composite-forming fiber-binder mixture to avoid undesirable shrinkage of the fibers. Also, the length of the fibers must not exceed the layer thickness of the composite since such excessively long fibers will cause deleterious alignment of the latter. For example, a composite with a layer thickness of mils requires the use of fibers of 15-mil length or less. Further, the lengths-of the fibers must be uniform to within about 10 percent of one another for preventing deleterious agglomeration from occurring in the fiber-resin composite. With fiber agglomeration it is impossible to provide random orientation and uniform dispersion of the fibers in the composite in the manner necessary for producing the desired physical properties described above. The length of the fibers found prac-' tical for most applications ranges from about 6 mils to 15 mils with a length-to-diamcter ratio of up to 400: 1. Also, the fibers are necessarily straight as practical or obtainable to prevent agglomeration or clumping together of the fibers.

The carbonizable binder with which the fibers are mixed may be a thermoplastic resin such as a coal tar or petroleumpitch or a thermosetting resin such as a phenolic, epoxy, furfuryl alcohol, and combinations of thermoplastic and/or thermosetting resins. Preferably, the resins selected for use in the fibrous composite are those which provide tenacious bonds with the individual fibers as well as a high coking factor, i.c., a high percentage of carbon or graphite yield when subjected to process temperatures in the temperature range described below. Normally,

It has been found that the fibrous composites may be fabricated by any one of several techniques including airspraying, molding, die-pressing, or a combinationof these techniques. Spraying is by far the preferred method of forming fibrous composites of various configurations since spraying facilitates the uniform dispersion of the fiberresin mixture on a configured form or surfaceand-also pro-motes uniform dispersion and random orientation of the individual fibers which are necessary for producing the I desired strength and ablation-resistant characteristics in the fibrous composites. I When air spraying a fiber-resin mixture onto a suitable form, the resin is preferably diluted with a suitable solvent or vehicle to facilitate the spraying operation. The

solvent should be one of the type which is readily evaporatable at temperatures substantially lower than the proc-' ess temperatures used for carbonizing the binder. Satisfactory results have been obtained by employing solvents such as acetone or methyl isobutyl ketone for phenolic resins and other thermosetting resins, and benzene and other suitable well known solvents for the thermoplastic resins. After the solvated or diluted resin-fiber mixture is sprayed onto the form, the binder is dried by evaporating the solvent from the mixture by air-drying, heating, or by employing any other suitable mechanism. A substantial quantity of the solvent evaporates during the spraying operation since only about 10 percent of the solvent is found in the sprayed composite.

The fibrous composites may be satisfactorily sprayed by using a commercially available recirculating-head spray gun with a -mil opening. The spray gun may be hand held, mounted on a movable support, or fixedly se cured in a standard spraying hood at a location suitably spaced from a rotating or otherwise movable form. Air ispreferably circulated around the form for drying thebinder, i.c., evaporating the solvent, as the fibrous mixture is deposited thereon. Before and during the spraying operation, the fiber mixture in the spray gun reservoir is thoroughly stirred to assure that the fibers are uniformly dispersed in the mixture. A satisfactory technique for stirring the mixture is the use of a conventional air-operated agitator.

The distance from the spray gun head to the surface of i sures may be more satisfactory when using other spray guns;

The fibrous carbonaceous composites may be of any" of a wide range of thicknesses, depending upon the envisioned usage of the composite. For example, the composite may be of a thickness ranging from a very thin layer of about 7 mils upwards to 2 inches or greater, if

desired. The fiber-resin mixture may be sprayed onto the form to produce the composite of the desircdthickness during a single spraying operation or, if desired, the composite may be of a laminated construction which is readily produced by spraying avplurality of thin layers. In the latter case, the binder in each layer is preferably dried andthermoset or cured before another layer is applied.

Fibrous composites may be prepared in a wide range of densities ranging from about 0.20 to 1.7 guts/cc. or more. Densities in the range of about 0.20 to 0.5 gm./cc. are readily provided by spraying the composite andcarbonizing at 1000 C. or graphitizing at 3000 C. without subjecting the composite to external stresses. In the formation of these low-density composites the 2-micron-diameter fibers usually provide the composites of the lower density, whereas 7-micron-diameter fibers are preferably employed for the higher-density composites. The density of the composites may be readily increased above the 0.2 to 0.5

gm./cc. range by stressing the mixture during the thermosetting or carbonizing steps, by impregnation with a diluted.

6 previously utilized. For example, the compressive strengths and tensile strengths are markedly greater in all three directions due to the random orientation and uniform 'dispersement of the fibers in the composite. Table III is illustrative of the strength properties provided by the fibrous composites of the present invention.

pitch, or by employing a pyrolytic graphite infiltration operation. Stressing the composite at pressures of about 2000 psi. during the thermosetting step has provided composite densities of about .94 gm./cc. by employing 2- and 7-micron-diameter fibers with a phenolic resin binder at a concentration of 100 p.p.h. However, composites stressed at pressures upwards to 12,000 psi. do not demonstrate marked differences over composites stressed at 2000 p.s.i. The higher-density materials, e.g., l.7 gms./cc., can be readily fabricated with either of'the 2-, 7-, or l0-micron-diarneter fibers by impregnating the time to the plasma, and the gross recession rate in inches 1 per second. The gross recession rate is the ratio of length composite with a liquidus thermoplastic or thermosetting change to composite dwell time.

TABLE IV Pie-test Post-test Change Change Gross in in Exposure recession Length, Weight, Length, Weight, length, weight, time, rate, inches grants inches grams inches grains secs. 1l\./. t0.

Not available lit-cause the collar around the sample could not be removed without damage to the sample.

resin, carbonizing the impregnatedcomposite, and then reimpregnating and carbonizing the composite until the desired density is obtained. The reimpregnation of the fibrouscomposite may be satisfactorily accomplished by employing well known evacuation or pressure techniques.

As shown in Tables 1 and 11, fibrous composites in a wide range of densities are formed without stressing and by using various stressing pressures and impregnations.

In order to provide a more facile understanding of the present invention. examples of preparing fibrous carbon or graphite composites are set forth below. in each of these examples the carbon or graphite fibers were initially prepared by chopping rayon yarn into the desired length and then carbonizing or graphitizing the rayon fibers at temperatures at least as great as the temperatures em- TAB LE I Binder thermo- Geometric setting or Number density Fiber Phenolic Stressing carbonizlng Cnrhonlzed of pitch alter unpreg diameter resin, pressure, temperature density, impreg- M10118. p.p.h. p.s.i. C. gmJcc. nations gms./cc

10 u... 12,000 0.80 10 1.61 E10 30, 000 150 0. 78 8 1. 73 30, 000 0. R0 7 l. 53 00 30, 000 150 0. 7 1 1. 02 100 12, 000 700 0.t|1 0 60 0 1, 000 0. .23 0

I At 12,000 p.s.l. the firstiucruaso in temperature caused hull'tho compact to extrutle out or a vent opening.

1 Three times with pitch to a density 011.28 gms./cc. Z Free standing on the graphite mandrel.

The fibrous composites of the present invention enjoy ployed in the binder carbonizing or graphitizing operaphysical properties which represent significant improvetion. Also, in each of these examples the spray gun merits over the fibrous composites prepared by techniques pressures and distances described above were employed.

7 EXAMPLE I Graphite fibers of -mil length and 7-micron diameter admixed with p.p.h. phenolic resin diluted with methyl isobutyl k'etone (1000 cc. diluent per 100 grams fiber) were sprayed onto a polytetrafiuoroethylene surface. T wo composites formed in this manner, one 0.1 inch thick and the other 0.3 inch thick, were thermoset without pressure at 150 C. and subsequently carbonized in an inert atmosphere (argon) at a temperature of 1000 C. Both composites were crack free, exhibited no shrinkage, and had a density of approximately 0.4 gram/cc.

EXAMPLE II Two composites of densities of 0.34 and 0.43 gram/cc. were prepared by compacting sprayed resin-fiber composites formed of graphite fibers of 2-micron diameter and 7-mil length which were admixed with a pitch binder (diluted with benzene) in concentrations of 100 and p.p.h., respectively. The sprayed composites were bagged in preformed polyvinyl chloride bags and evacuated, then preheated in an ovento 150 C. for 8 hours and thereafter isostatically pressed in cold oil at 5000 p.s.i. The pressure was allowed to remain on the composites until the temperature of the latter dropped to approximately 30 C. This procedure reduced the thickness of the composites to about one-half. After the pressing operation, the composites were carbonized and the thicknesses increased approximately 20 percent from the pressed condition. As a result of this pressing the carbonized density was approximately doubled over that normallyachieved by-carbonizing 'without pressing.

EXAMPLE III Graphite fibers were admixed with a phenolic binder (diluted with methyl isobutyl ketone) at a concentration of 90 p.p.h. and sprayed onto a form to a final thickness EXAMPLE IV A fibrous graphite composite was prepared by mixing grams of graphite fibers (6 mils in length and 2 microns in diameter) with phenolic resin at 100.p.p.h. and diluted with 1000 cc. of methyl isobutyl ketone. The

admixture was sprayed onto a polytetralluoroethylenesubstrate, thermoset without pressure at C., and then carbonized at 900 C. and subsequently graphitizecl at 3000 C. The composite had a final density of 0.22 gram/cc.

The preceding examples are merely illustrative of the fibrous composites producible in accordance 'with the teachings of the present invention and should not be construed as limitations since various modifications can be made in the method and product described herein without departing from the spirit and scope of the present invention. All such modifications are within the scope of the appended claims and are considered as part of the present invention.

What is claimed is:

1. A fibrous carbonaceous product comprising a matrix,

consisting of a resin converted to a carbonaceous state and a plurality of virtually straight carbonaceous fibers uniformly dispersed in and randomly oriented throughout said matrix, said fibers being of lengths within about 10 percent of one another in a range of about 6 mils to about 15 mils and of a diameter in a. range of about 1 micron to about 10 microns.

2. The fibrous carbonaceous product claimed in claim 1, wherein the carbonaceous state of said matrix is carbon, and wherein the carbonaceous fibers consist of at least one of carbon and graphite.

3. The fibrous carbonaceous product claimed in claim 1, wherein the carbonaceous state of said matrix is graphite, and wherein the carbonaceous fibers consist of graph--.

ite

1, wherein the product has a density in the range of about 0.20 gm./cc. to about 1.7 gms./cc., wherein the product has a thickness in the range of about 7 mils to greater.

than about 2 inches, and wherein the length of said fibers does not exceed the thickness of said product.

5. A process for manufacturing a formed carbonaceous composite incorporating randomly oriented and uni-' formly dispersed carbonaceous fibers, comprising the steps of forming an admixture of a carbonizable binderand virtually straight carbonaceous fibers of lengths within about 10 percent of one another in a range of about 6 mils to about 15 mils and of a diameter in a range of about 1 micron to about 10 microns, spraying the admixture onto a form, and thereafter heating the sprayed admixture to a temperature sufficient to convert the binder to a carbonaceous state, said fibers being subjected .to a

selected from the group consisting of thermoplastic resins, g I

thermosetting resins, and combinations of said resins.

7. The process for manufacturing a formed carbonaceous composite as claimed in claim 5, wherein the carbonaceous fibers are in the form of graphite, the carbonizable binder is selected from the group consisting of thermoplastic resins, thermosetting resins, and combinations of said resins, and wherein said first-mentioned temperature is sufiicient to'convert the carbonizable binder to graphite.

8. .The. process for manufacturing a formedcarbonaceous composite as claimed in claim 5, including the additional step of curing the carbonizable binder prior to heating the sprayed admixture to said first-mentioned temperature, and stressing the sprayed admixture during at least one step defined by the curing of the carbonizable binder and the conversion of the latterto a carbonaceous state.

9. The process for manufacturing a formed carbonaccQ ous composite as claimed in claim 5, including the steps of impregnating the composite with a liquid'us carboniz-- able resin subsequent to the conversion of the binder to a carbonaceous state, and thereafter heating the impreg-' nated composite to a temperature sufficient to convert the carbonizable resin to a carbonaceous state.

10. The process for manufacturing a formed carbonaceous composite as claimed in claim 5, including the additional steps of sutficiently diluting the carbonizable binder with a solvent therefor to effect said spraying and driving the solvent from the sprayed admixture prior to the conversion of the binder to a carbonaceous state.

11. The process for manufacturing a formed carbonaceous composite as claimed in claim 10, wherein the ALFRED L. LEAVITT, Primary Examiner E. G. WHITBY, Assistant Examiner US. Cl. X.R.

4. The fibrous carbonaceous product claimed in claim 

